1. Field of the Invention.
This invention relates to determining the position of a remote object with respect to a reference point; and, more particularly, to radiating an electromagnetic field from the reference point, detecting the field at the remote object and analyzing the detected field to determine the position of the remote object.
2. Description of the Prior Art
The use of orthogonal coils for generating and sensing magnetic fields is well known. Such apparatus has received wide attention in the area of mapping magnetic fields to provide a better understanding of their characteristics, for example. If a magnetic field around generating coils can be very accurately mapped through use of sensing coils, it has also been perceived that it might be possible to determine the location of the sensing coils relative to the generating coils based on what is sensed. However, a problem associated with doing this is that there is more than one location and/or orientation within a usual magnetic dipole field that will provide the same characteristic sensing signals in a sensing coil. In order to use a magnetic field for this purpose, additional information must therefore be provided.
One approach to provide the additional information required for this purpose is to have the generating and sensing coils move with respect to each other, such as is taught in U.S. Pat. No. 3,644,825. The motion of the coils generates changes in the magnetic field, and the resulting signals then may be used to determine direction of the movement or the relative position of the generating and and sensing coils. While such an approach removes some ambiguity about the position on the basis of the field sensed, its accuracy is dependent on the relative motion, and it cannot be used at all without the relative motion.
Another approach that has been suggested to provide the additional required information is to make the magnetic field rotate as taught in Kalmus, "A New Guiding and Tracking System," IRE Transactions on Aerospace and Navigational Electronics, March 1962, pages 7-10. To determine the distance between a generating and a sensing coil accurately, that approach requires that the relative orientation of the coils be maintained constant. It therefore cannot be used to determine both the relative translation and relative orientation of the generating and sensing coils.
U.S. Pat. No. 3,868,565 teaches a tracking system for continuously determining at the origin of a reference coordinate system the relative translation and orientation of a remote object. The tracking system includes radiating and sensing antenna arrays each having three orthogonally positioned loops. Properly controlled excitation of the radiating antenna array allows the instantaneous composite radiated electromagnetic field to be equivalent to that of a single loop antenna oriented in any desired direction. Further control of the excitation causes the radiated field to nutate about an axis denoted a pointing vector.
The tracking system is operated as a closed loop system with a computer controlling the radiated field orientation and interpreting the measurements made at the sensing antenna array. That is, an information feedback loop from the sensing antenna array to the radiating antenna array provides information for pointing the axis of the nutating field toward the sensing antenna array. Accordingly, the pointing vector gives the direction to the sensing antenna array from the radiating antenna array. The proper orientation of the pointing vector is necessary for computation of the orientation of the remote object. The signals detected at the sensing antenna include a nutation component. The nutating field produces a different nutation component in each of the three detected signals. The orientation of the sensing antenna array relative to the radiated signal is determined from the magnitudes of these components.
While the art of determining position and orientation of remote objects is a well developed one, there still remains a need to determine the relative position of a remote object with respect to a reference coordinate frame without imposing movement and orientation constraints on the remote object or the radiated electromagnetic field. Further, there is a need for continuously and simultaneously determining at a plurality of remote objects the relative positions of the remote objects with respect to a reference coordinate frame.